The present invention relates to a planar waveguide dispersion compensator for an optical signal, and to a method for compensating for dispersion in an optical signal. Particularly, but not exclusively, the invention further relates to a thermally responsive lens providing dispersion compensation in a planar waveguide device, and to a method of tuning a thermally responsive dispersion compensator.
Digital optical transmission systems such as glass fiber pulse code modulation (PCM) transmission systems are known to suffer from chromatic (wavelength dependent) dispersion. Such dispersion leads to optical signals propagating along a fiber or within a planar waveguide being subject to delays in their propagation time which depend on their wavelength. In this document, the term planar waveguide refers to an optical waveguide which is provided in a substantially integrated form such as in a planar light circuit, and which comprises a light-guiding region supported by a suitable substrate for example, a silicon or silica type substrate. In particular, the term planar waveguide encompasses a thin strip or film of material having a relatively higher refractive index which is embedded in the surface of a planar or laminar substrate.
The variable delay which chromatic dispersion generates in optical communications networks creates several problems, especially in digital optical transmission systems. As transmission rates increase in digital optical communications networks, providing cheap, reliable and efficient means to implement dispersion compensation and to control the pulse profile of an optical signal during transmission through optical media is highly desirable. By reducing the amount of dispersion in an optical signal higher bit rates can be more reliably accommodated.
The theoretical approach to preventing spread in a digital signal during transmission involves compensating for the variations in phase that arise from a frequency dependent group velocity in the transmission system.
Two ways in which a system may be constituted to compensate for dispersion are adding a length of line, for example an additional length of optical waveguide, of opposite dispersion characteristics to the previous portion of the line or applying a suitable phase-versus-frequency characteristic to the signal. Consider the case where a spectral component of a signal propagating along line 1 of length z1 has angular frequency xcfx89. The spectral component has a propagation constant xcex21 along line 1. Along an additional length of line, line 2 of length z2, the spectral component has a propagation constant xcex22. Either propagation constant xcex21, xcex22 may be frequency dependent. If the initial, arbitrary, phase is xcfx860, then the phase at output is xcfx861=xcfx89t+xcfx860xe2x88x92xcex21z1xe2x88x92xcex22z2.
The change of phase at a given frequency deviation xcex4xcfx89 from the center frequency is given by {t-(dxcex21/dxcfx89)z1xe2x88x92(dxcex22/dxcfx89)z2xe2x88x92xcex22(dz2/dxcfx89)}xcex4xcfx89. To prevent distortion of the signal, the phase variation should remain zero over the whole range of frequencies contained within it. As the dxcex2/dxcfx89 and dz/dxcfx89 terms can vary over the frequency range, it is necessary that the second derivative with respect to frequency is also zero giving:                               (                                    d              2                        ⁢                                          β                1                            /              d                        ⁢                          xe2x80x83                        ⁢                          ω              2                                )                ⁢                  Z          1                            (        1        )              +                            (                                    d              2                        ⁢                                          β                2                            /              d                        ⁢                          xe2x80x83                        ⁢                          ω              2                                )                ⁢                  z          2                            (        2        )              +                            2          ⁢                      (                          d              ⁢                              xe2x80x83                            ⁢                                                β                  2                                /                d                            ⁢                              xe2x80x83                            ⁢              ω                        )                                    (          3          )                    ⁢              (                                            dz              2                        /            d                    ⁢                      xe2x80x83                    ⁢          ω                )              +                            β          2                ⁡                  (                                    d              2                        ⁢                                          z                2                            /              d                        ⁢                          xe2x80x83                        ⁢                          ω              2                                )                            (        4        )              =  0
The above equation shows three ways that are available for compensating group delay distortion in a fixed length z1 of line 1 represented by term (1). Firstly, term (2), can provide compensation by adding line 2 of length z2 of opposite group velocity dispersion. Secondly, term (3) can provide compensation when the length z2 of line 2 is linearly dependent on the frequency. Thirdly, term (4) can provide compensation when the length z2 of line 2 is a strongly quadratic function of frequency and dominates term (3).
A number of factors influence dispersion and delay and it is not easy to compensate in a planar waveguide device for the group velocity dispersion of the transmission system through which an optical pulse propagates. Although polymer materials can provide a compensating dispersion of group velocity, such materials are generally considered unsuitable for pulse reforming due to size constraints in a planar waveguide device. An optical pulse needs to have a relatively long propagation path within the polymer material to ensure that a sufficiently large compensating group delay dispersion is induced.
Another way to induce a compensating group delay dispersion for a signal is to linearly change the path-length of each component signal of a pulse to induce a sufficient relative change in phase with respect to the relative wavelength difference between the component signals. This is described by term (3) in the equation and can be achieved in non-planar optical environments for example, by using an adjustable chirped grating.
Conventional dispersion compensators using techniques such as stretchable chirped fibre gratings to alter the refractive index of the fibres implementing the grating are generally complex, expensive, and are subject to fatigue.
The invention seeks to obviate or mitigate the above problems by providing a dispersion compensator for an optical signal.
A first aspect of the invention seeks to provide a dispersion compensator for an optical signal comprising: optical signal input means to receive said optical signal as input; optical signal decomposing means connected to said input means and arranged to decompose the optical signal into a plurality of component signals, each component signal having a different passband from an adjacent component signal; optical dispersion means having an optical path-length adjuster arranged to receive each said component signal with an initial phase and configured to adjust the optical path length of at least one said component signal to induce a phase shift in said component signal on output; and an optical signal combiner arranged to re-combine the component signals output by said path-length adjuster into a re-combined signal, wherein the phase shift of each component signal is selected to correct in the recombined signal any dispersion present in the inputted optical signal.
Advantageously, the optical signal decomposition means is able to resolve said component signals sufficiently for said induced relative phase shift to provide a satisfactory level of dispersion compensation.
Advantageously, the invention enables a digital optical signal to receive compensation for any dispersion.
Preferably, said optical signal decomposing means comprises a first array of M waveguides and said optical dispersion means comprises a second array of N waveguides and said compensator further includes: a first 1:M coupler connected to said signal input means and splitting said inputted optical signal along said first array of waveguides; and a second M:N coupler connected to said first array of waveguides and to said second array of waveguides and arranged to decompose optical signals from said first array of waveguides into said component signals.
Preferably, said path-adjuster comprises at least one lens having a refractive index which is capable of differing from the refractive index of the waveguide along which a component signal is propagating.
Preferably, said dispersion compensator according to said first aspect is provided as a planar waveguide device, wherein the path-adjuster comprises at least one strip lens embedded in a first layer of said waveguide device, wherein each said strip lens has a refractive index which is capable of differing from the refractive index of the waveguide along which a component signal is propagating, wherein a heat channeling element is provided in a second layer below said first layer.
Preferably, the induced phase shift of each component signal is a quadratic function of the wavelength of each component signal.
Preferably, the signal combiner comprises said first coupler, the compensator further comprising a reflector arranged to reflect phase-shifted component signals back along their incident optical paths.
Alternatively, the signal combiner further includes: a N:P coupler connected to said path length adjuster and to a third array of P waveguides; and a P:1 coupler connected to said third array of waveguides and arranged to combine the phase shifted component signals into a single signal.
Preferably, the path length adjuster has at least one thermal characteristic affecting the path-length of at least one component signal, and the dispersion compensator further includes thermal control means controlling the path adjustment means.
Preferably, the dispersion compensator according to said first aspect further includes a polarization adjuster to adjust the polarization of the component signals.
A second aspect of the invention seeks to provide a method of compensating for dispersion in an optical signal comprising the steps of: decomposing the optical signal into component signals having differing passbands; inducing a phase-shift in each component signal by adjusting the optical path of each component signal relative to each other; and combining component signals into a combined optical signal, wherein the induced phase shift is selected, to provide a dispersion correction in said combined signal.
Preferably, the method according to the second aspect of the invention further includes the step of selecting the induced phase shift for each component signal to be a quadratic function of the wavelength of each component signal.
Preferably, the method according to the second aspect of the invention further includes the step of selecting the induced phase shift of each component signal to adjust the width of a pulse profile of the combined optical signal relative to the initial optical signal.
Preferably, the method according to the second aspect of the invention further includes the step of adjusting the phase of each component signal using thermally dependent path-length adjusting means to adjust the relative path-length of the component signals.
A third aspect of the invention seeks to provide an optical transmission system including a dispersion compensator for an optical signal, the compensator comprising: optical signal input means to receive said optical signal as input; optical signal decomposing means comprising at least one array of waveguides connected to said input means and arranged to decompose the optical signal into a plurality of component signals, each component signal having a different passband from an adjacent component signal; optical dispersion means having an optical path-length adjuster arranged to receive each said component signal with an initial phase and configured to adjust the optical path length of at least one said component signal to induce a phase shift in said component signal on output; and an optical signal combiner arranged to re-combine the component signals output by said path-length adjuster into a re-combined signal, wherein the phase shift of each component signal is selected to correct in the recombined signal any dispersion present in the inputted optical signal.
A fourth aspect of said invention seeks to provide a path length adjuster for a dispersion compensator, the path length adjuster comprising a plurality of planar waveguide strip lens, the strip lens comprising: a middle portion of substantially uniform thickness; and at least one end portion having a different thickness from said middle portion.
Preferably, at least one end portion of at least one strip lens has a stepped profile.
A fifth aspect of the invention seeks to provide a dispersion compensator for a pulsed optical signal comprising: an optical signal decomposer arranged to separate an inputted optical signal into a plurality of component signals having different passbands and optical paths; a path length adjustor arranged to adjust the optical path length of each component signal by a pre-determined amount; and an optical signal combiner to recombine said optical path-adjusted signals into a recombined optical signal, wherein the amount of optical path length adjustment is sufficient to provide a dispersion correction to said recombined optical signal.
A sixth aspect of the invention seeks to provide a dispersion compensator for a pulsed optical signal including: an optical signal decomposer arranged to separate an inputted optical signal into a plurality of component signals having different passbands and optical paths; a temperature responsive path length adjuster arranged to adjust the optical path length of each component signal by a pre-determined amount, temperature control means for said path length adjuster arranged to control the temperature of said path length adjuster; and means to recombine said optical path-adjusted signals into a recombined optical signal, wherein the amount of optical path length adjustment is sufficient to provide a dispersion correction to said recombined optical signal.
A seventh aspect of the invention seeks to provide a dispersion compensator for a pulsed optical signal comprising: an optical signal decomposer arranged to separate an inputted optical signal into a plurality of component signals having different passbands and optical paths; a path length adjustor arranged to adjust the optical path length of each component signal; and an optical signal combiner to recombine said optical path-adjusted signals into a recombined optical signal, wherein the optical path length adjustment provides a dispersion correction to said recombined optical signal.
An eighth aspect of the invention seeks to provide a planar waveguide dispersion compensator for a pulsed optical signal including, an optical signal decomposer provided within a silica layer of said planar waveguide and arranged to separate an inputted optical signal into a plurality of component signals having different passbands and optical paths; a temperature responsive path length adjuster arranged to adjust the optical path length of each component signal by a predetermined amount and provided in said silica layer; temperature control means for said path length adjuster arranged to control the temperature of said path length adjuster; a heat channeling element arranged to increase the amount of heat flowing from said temperature control means to said path length adjuster; and means to recombine said optical path-adjusted signals into a recombined optical signal, wherein the amount of optical path length adjustment is sufficient to provide a dispersion correction to said recombined optical signal.
A ninth aspect of the invention seeks to provide a method of thermally tuning a dispersion compensator according to any appropriate aspect of the invention, such as are apparent to a person skilled in the art, for example, the eighth aspect comprising the steps of:
a) thermally tuning said passbands of said decomposed signals; and
b) thermally tuning said path length adjuster to adjust the optical path length to provide a desired level of dispersion correction.
Other aspects of the invention as set fourth in the priority document include providing a dispersion compensator for an optical signal comprising: an arrayed waveguide grating (AWG) having a number M of waveguides, the AWG decomposing the optical signal into N component signals each having a separation wavelength from an adjacent component signal; at least one path-length adjustment means varying the path-length of at least one of said N component signals to induce a phase shift between the initial phase of each component signal and the final phase of each component signal; and recombination means to re-combine the phase-shifted component signals into a re-combined signal, wherein the phase shift of each component signal is selected to adjust at least one characteristic of the optical signal in the recombined signal.
The dispersion compensator may further include an M:N coupler, wherein the arrayed waveguide grating is connected to the M:N coupler such each of the N component signals is carried along one of N waveguides.
At least one path-adjuster may comprise at least one lens having a refractive index which is capable of differing from the refractive index of a waveguide along which a component signal is propagating. At least one path-adjuster preferably comprises at least one strip lens having a refractive index which is capable of differing from the refractive index of a waveguide along which a component signal is propagating, and wherein at least one strip lens is thicker at either end than in a middle portion. Preferably, at least one characteristic is a group delay of the optical signal. Preferably, the phase shift xcex94xcfx86 of each component signal is a quadratic function of the wavelength of each component signal. At least one characteristic of the optical signal adjusted is preferably a width of a pulse profile of the optical signal. The phase shift xcex94xcfx86 of each component signal is preferably determined to induce an appropriate dispersion compensating group delay for the re-combined signal. Preferably, the recombiner comprises: a reflector capable of reflecting the phase shifted component signals; the reflector being provided so that the phase shifted component signals return along their incident paths. For example, the reflector may be a mirror or a partially silvered mirror(s). The recombiner may include a N:M coupler; an arrayed waveguide having a number M of waveguides, and M:1 coupler provided to combine the phase shifted component signals into a single signal. The path length adjuster may have at least one thermal characteristic affecting the path-length of at least one component signal, and the dispersion compensator may further include thermal control means controlling the path adjustment means. The dispersion compensator may further include a polarisation adjuster to adjust the polarisation of the component signals.
The dispersion compensator thus advantageously enables an optical signal which has undergone dispersion to be narrowed within an optical medium. By providing such a dispersion compensator as a planar waveguide device, the dispersion compensator is compact and easily integrated into optical components.
Another aspect seeks to provide a method of compensating for dispersion in an optical signal comprising the steps of: decomposing the optical signal into component signals which differ from each other by a fractional wavelength xcex94xcex; adjusting the phase of each component signal by an induced phase shift xcex94xcfx86; and recombining each component signals into a re-combined signal, wherein the phase shift xcex94xcfx86 is selected to adjust at least one characteristic of the optical signal in the re-combined signal.
The method may further comprise the step of selecting the induced phase shift xcex94xcfx86 to determine a group delay dispersion of the re-combined signal. Preferably, the method further includes the step of selecting the phase shift xcex94xcfx86 to provide a different group delay dispersion for the re-combined signal to the initial group delay dispersion of the optical signal. Preferably, the method further includes the step of selecting the phase shift xcex94xcfx86 of each component signal to induce zero group delay dispersion in the re-combined signal. The method may further include the step of selecting the phase shift xcex94xcfx86 of each component signal to be a function of the wavelength of each component signal. The method may further include the step of selecting the phase shift for each component signal to be a quadratic function of the wavelength of each component signal. The method may further include the step of selecting the phase shift of each component signal to adjust the width of a pulse profile of the optical signal. The method may further include the step of adjusting the phase of each component signal using thermally dependent path-length adjusting means to adjust the relative path-length of the component signals. The method may further include the step of adjusting the polarisation of each component signal.
Other aspects seek to provide an optical component including a dispersion compensator according to any appropriate aspects of the invention, a node for an optical network including a dispersion compensator according to any appropriate aspects of the invention, and an optical transmission system including a dispersion compensator according to any appropriate aspect of the invention.
Other aspects also include a planar waveguide dispersion compensator for an optical signal which applies a phase shift xcex94xcfx86 to the optical signal, where the phase shift xcex94xcfx86 is a function of the wavelength of the optical signal, and wherein the phase shift xcex94xcfx86 is selected to adjust at least one characteristic of the optical signal in the re-combined signal. Any features of the above features may be suitably combined and/or incorporated in any of the above aspects as would be apparent to a person skilled in the art. Moreover, terms such as adjuster are to be construed to include appropriate equivalents capable of acting as an adjuster as would be obvious to those skilled in the art. Similarly, terms such as re-combiner are to be construed to include appropriate equivalents capable of acting as a signal recombiner.
The invention thus provides a planar dispersion compensator for an optical signal. The compensator decomposes an inputted optical signal into N component signals separated by a fractional wavelength xcex94xcex. Each component signal has its path-length adjusted to induce a sufficient phase shift between input and output to change the group delay dispersion of the optical signal when recombined from each of the component signals. This behaviour is described by term (4) in the equation presented herein above. In this manner, pulse broadening can be compensated by selectively varying the induced phase shifts to produce the desired level of opposite group delay dispersion.
Advantageously, the dispersion compensation mechanism provides a means of inducing a group delay dispersion opposite to that of an optical signal in a relatively compact area. This is particularly advantageous in optical networks which carry traffic at high transmission rates. In any high-bit rate environment it is highly advantageous to be able to compensate signal dispersion in a reliable and compact manner.
By compensating for dispersion in the optical layer, both passive or active dispersion compensation can be implemented i.e. the amount of compensation may be predetermined (passive) or actively adjusted. Another advantage of the invention is that the invention can be implemented in a planar optical device.
The invention enables digital optical signal processing which comprises one or more instances of apparatus embodying the present invention, together with other additional apparatus.
By using the differential thermal response of different materials in a planar AWG, the mechanical strain/stress mechanisms such as stretchable chirped fibre gratings employ can be avoided.